1. Field of the Invention
The present invention relates to an organic EL (electro-luminescent) element and organic EL display.
2. Description of the Related Art
Along with the recent development of the information society, various kinds of mobile devices and terminal devices are increasingly becoming popular. Demand for reduction of power consumption in displays mounted on the devices is growing ever.
An organic EL element is a self-emission element and constitutes a light-emitting diode in which an organic layer including a light-emitting layer is sandwiched between a cathode and an anode.
The organic EL element can emit light upon application of a low voltage of 10 V or less. The organic EL element can also realize various emission colors including three colors: blue, green, and red. From these viewpoints, organic EL displays have received a great deal of attention as flat panel displays of next generation that should replace liquid crystal displays. However, many organic EL displays reduce by half the luminance within 10,000 hrs. That is, no current organic EL displays have a sufficient long panel life, unlike liquid crystal displays.
A luminance half-life τ of an organic EL element is closely related to its drive current density J necessary for obtaining a desired panel front luminance L on an organic EL display. As is experimentally known, the luminance half-life τ can be expressed byτœ1/J=η/L  (1)where η is the luminous efficiency of the organic EL element.
As is apparent from equation (1), the higher the drive current density becomes, the faster luminance degradation progresses. To increase the reliability or life of the organic EL element while maintaining the luminance L at a predetermined value, the luminous efficiency η should be increased.
The luminous efficiency η of the organic EL element is given byη=Φe-h×ΦR×ΦOUT  (2)where Φe-h is the electron/hole injection balance, ΦR is the recombination radiation efficiency of the light-emitting layer material, and ΦOUT is the outcoupling efficiency. ΦR is a value determined by the light emission capability of each of the R, G, and B light-emitting materials. ΦOUT is a value determined by the three-dimensional structure of the device. To increase the luminous efficiency η without changing the materials and device structure, it is effective to improve the electron/hole injection balance Φe-h.
To increase the electron/hole injection balance Φe-h, generally, the cathode structure is optimized. For example, Jpn. Pat. Appln. KOKAI Publication No. 10-74586 describes, as the cathode of an organic EL element 30, a two-layered cathode having an Y/X structure including a contact layer (referred to as an X layer hereinafter) 32x and a cathode conductor layer (referred to as an Y layer hereinafter) 32y, as shown in FIG. 1. Referring to FIG. 1, reference numeral 31 denotes an anode; and 33, an organic layer. Jpn. Pat. Appln. KOKAI Publication No. 2000-164359 describes a three-layered cathode having an M/Y/X structure further including a protective conductor layer (referred to as an M layer hereinafter) 32m on the Y layer, as shown in FIG. 2.
In an Y/X cathode 32 shown in FIG. 1, Al is used as the material of the Y layer 32y, and LiF is used as the material of the X layer 32x. This Al/LiF cathode is a typical cathode used in an organic EL element including a low molecular light-emitting layer. The Al/LiF cathode has a great electron injection ability for an Alq3 electron transporting layer used in combination with the low molecular light-emitting layer. In this structure, Φe-h up to 1 has been reported. However, the electron injection ability of the Al/LiF cathode is low for a low molecular electron transporting layer other than Alq3 or a polymer light-emitting layer. According to studies by the present inventors, the electron injection barrier height of the cathode interface, which determines the electron injection amount, largely depends on the type of material used for the X layer 32x. When a polymer light-emitting layer and a LiF layer is combined, the barrier is high, and electrons are hardly injected. For this reason, when an Al/LiF cathode is used, no high luminous efficiency η can be realized in a polymer organic EL element or a low molecular organic EL element having an electron transporting layer other than Alq3. Hence, the life is short.
On the other hand, in the M/Y/X cathode 32 shown in FIG. 2, Al which is stable and is hardly oxidized is used as the material of the M layer 32m, Ca is used as the material of the Y layer 32y, and LiF is used as the material of the X layer 32x. This Al/Ca/LiF cathode is a typical cathode used in a polymer organic EL element. The Al/Ca/LiF cathode 32 has a structure obtained by adding a Ca layer with a small work function to the above-described Al/LiF cathode 32. Addition of the Ca layer is supposedly done to decrease the electron injection barrier height of the cathode interface to increase the electron injection amount. However, even when the cathode 32 is used, the electron injection ability for a polymer light-emitting layer cannot sufficiently be improved. For this reason, even when an Al/Ca/LiF cathode is used, no high luminous efficiency η can be realized in a polymer organic EL element or a low molecular organic EL element having an electron transporting layer other than Alq3, as in the Al/LiF cathode. Hence, the life is short. In addition, the Al/Ca/LiF cathode has a structure obtained by adding a highly chemically reactive Ca layer to the Al/LiF cathode. For this reason, Ca atoms are diffused to the Al layer and LiF layer as time elapses. Accordingly, the life shortens due to degradation of the cathode.